U.S. Pat. No. 4,790,743 describes a staged combustion method, in which the flame zones are divided into a primary flame zone which is formed of a supply of coal dust and its carrier air, and the air to fuel ratio is in the range of 0.65-0.9; adjacent to the primary flame zone is the secondary flame zone formed of a supply of reduction fuel injected via reduction fuel nozzles, having a lower air to fuel ratio in the range of 0.5-0.8; and in the final flame zone which is formed of a supply of staged air injected through stage air nozzles, the ratio being in the range of 1.05-1.4. The primary flame zone has a strong internal back flow region due to the arrangement of an adjustable swirling apparatus and a flared burner mouth so as to maintain a high ignition stability.
U.S. Pat. No.4,629,413 discloses a premix burner with stage combustion, in which the burner consists of a primary air-fuel gas combustion assembly and a secondary combustion assembly. The primary combustion assembly has a mixer for premixing fuel gas and the primary air; the secondary combustion assembly has a number of secondary air ports and secondary air inlet means therefor to delay the contact of the secondary air with the main flame so as to reduce the formation of NOx.
In U.S. Pat. No. 4,708,6388, Brazier et al. teaches a method in which the primary combustion air and the secondary combustion air respectively supplied via the primary and secondary air inlets are simultaneously fed into the burner and, by means of a swirler, the primary combustion air, the secondary combustion air, the induced flue gas, and the fuel injected from the nozzle are stirred and mixed in the flame zone.
All of the patents discussed above fail to provide an effective method to maintain the burning intensity and flame stability in the primary flame zone. Also, the mixing of air and fuel in the primary flame zone is not very satisfactory.